Pleomorphic Lobular Carcinoma of the Mammary Gland in Women and Female Dogs: A Comparative Clinical-Pathological and Immunophenotypic Analysis.

Pleomorphic invasive lobular carcinoma (PILC or PLC) is a malignant breast tumor considered a rare variant of invasive lobular carcinoma in women, characterized morphologically by marked nuclear pleomorphism, with cells resembling plasmacytoid, histiocytoid, or apocrine cells. One of its defining features is the loss of E-cadherin expression. Considering the biological similarities between species and the limited data available for female dogs, this study aimed to characterize PLC in canines, with an emphasis on its histopathological and immunophenotypic features and its potential applicability as a comparative model. Histopathological analysis of PLC was performed alongside immunohistochemical evaluation using HER2, estrogen receptors (ER), progesterone receptors (PR), E-cadherin, and Ki-67 (cell proliferation indexing) markers. All canine PLCs tested positive for PR, with the majority being negative for ER, and all were negative for HER2 and E-cadherin. In contrast, in women, all cases were positive for ER, most were positive for PR, and all were negative for HER2. The Luminal B molecular subtype was the most frequent in dogs, whereas Luminal A and Luminal B subtypes showed equal prevalence in women. These findings reveal shared and distinct immunophenotypic features between species. The similarities and differences observed emphasize the relevance of the canine model for comparative oncology. Furthermore, the use of spontaneous tumors in immunocompetent dogs in this study strengthens the translational potential of the findings, thereby reinforcing the use of the canine model in breast cancer research and supporting its role within the One Medicine concept.